Recovery of polyolefins using a mixture of alcohol and stannic chloride



United States Patent Ofi ice 3,192,1QZ Patented June 29, lififiS 3,192,192 RECOVERY OF POLYOLEFINS USING A MEiTURE OF ALCOHOL AND STANNZC CHLORIDE Robert L. Hartnett, Texas City, Tex., assignor to Monsanto Company, a corporation of Delaware N Drawing. Filed July 20, 1962, Ser. No. 211,443

7 Claims. (Cl. 26093.7)

This invention relates to the polymerization of olefins. More particularly, it relates to an improved method for separating olefin polymers from mixtures produced by the polymerization of olefins in the presence of so-called l V Max Planck Institute at Mulheim, Ruhr, Germany, and

has come to be commonly termed the Ziegler process, while the catalysts employed are commonly referred to as Ziegler catalysts. Many variations of the catalyst have been disclosed but probably the preferred groups are those described in Belgian Patents Nos. 533,362 and 534,792, the disclosures of which are incorporated herein by reference, namely, catalysts prepared by the interac tion of strong reducing agents such as organometallic compounds like metal alkyls with a compound of a metal of Groups IV-B, V-B, and VI-B of the periodic table of the elements including thorium and uranium and especially the halides of titanium and zirconium.

As the process is ordinarily conducted, the catalyst is suspended in a suitable liquid organic medium or inert solvent such as hexane, benzene, and the like, and the olefin is bubbled through the system at a temperature anywhere in the range from 0 to 100 C. and at relatively low pressures. Upon completion of the reaction, the polymer is recovered by first destroying the catalyst by contacting or quenching it with an alcohol and then separating the precipitated polymer from the reaction mixture by filtration or centrifugation. After the polymer is removed from the reaction mixture, it is usually washed with a suitable solvent in order to remove final traces of the catalyst residues and the reaction medium. Alkyl alcohols can be used for this purpose. After washing, the polymer is dried by any convenient method.

The olefin polymers obtained have superior and highly desirable properties. Examples of particularly valuable solid polymers produced by the Ziegler process are polyethylene and polypropylene, especially the so-called isotactic or crystalline polypropylene However, the Ziegler catalysts have the disadvantage of leaving metallic residues intimately admixed with, occluded in, and perhaps, bonded to the polymer. The presence of these metallic residues tends to impart undesirable characteristics to the polymer. They lead, for example, to degradation and discoloration of the polymer when it is heated or when it is exposed to light, particularly daylight. When subsequently molded, melt fabricated, or otherwise shaped in the presence of heat, the polymers become darker than is desirable and consequently their utility is seriously restricted. While it has been found that antioxidants and like compounds may sometimes be employed to stabilize organic materials "against the effects of degradation induced by exposure to heat and light, the usefulness of such materials is often greatly diminished or even nullified in the presence of such metallic oxidation catalysts as are found in the catalyst residues remaining in the "polymers produced by the Ziegler process. In addition to detrimental color effects arising from molding, the electrical properties, particularly the electrical insulating properties of the molded products, are also adversely affected. Further, the catalyst residues retained in the polymer will cause corrosion of process equipment employed in the fabrication of the polymer. Thus, it will be seen that it is imperative to remove these metal-containing catalyst residues from the polymers.

This need is met generally in the prior art by quenching of the catalyst in the reaction medium with an alcohol. Alcoholysis will cause dissolution of the metal-topolymer bond and effect solvolysis of the metal. However, in the usual alcohol treatment only the dissolved fraction or the solid particles of the catalyst residue which are soluble in the alcohol are affected. There thus remains a need for further improvements in the purification of olefin polymers to remove remaining residues of the metallic catalysts in order to improve the quality of the resulting product. Various after-treatments of the polymer have been proposed for this purpose also but the employment of an agent which can be incorporated in the quenching step wherein the bulk of the catalyst is destroyed has obvious advantages over these.

Now in accordance with the present invention, it has been found that metallic catalyst residues obtained in the Ziegler polymerization of olefins can be effectively reduced by a process which comprises contacting the catalyst in the reaction mixture after the polymerization has taken place and before any substantial removal of the polymer from the reaction mixture with an alkyl alcohol and a minor amount of a stannic halide. Polymers produced by this process are less subject to deterioration and discoloration during shaping and forming operations than those produced using conventional quenching techniques.

Any alkyl alcohol containing from 1 to 8 carbon atoms can be used. Of the suitable alcohols, methyl alcohol, ethyl alcohol, propyl alcohol, isopropyl alcohol, n-butyl alcohol, tert-butyl alcohol, isobutyl alcohol, amyl alcohol, hexyl alcohol, octyl alcohol, and the like, methyl alcohol is the preferred quenching agent. The amount of alcohol used for quenching is not critical and may be varied widely from about 1% to about 300% of the weight of the polymer slurry being treated. Generally, amounts from about 5% to about 25% by weight of the polymer slurry are satisfactory but the amount can be controlled as desired to provide an amount of alcohol suflicient to form a slurry of satisfactory fluidity.

While stannic bromide and stannic iodide can be employed in the practice of the invention, stannic chloride is the preferred treating agent. The amount of the stannic halide employed with the alcohol depends to some extent upon the catalyst employed, catalyst concentration, the desired purity of the polymer, and the l ke. Usually, amounts from about 0.5 to about 5% by weight of the slurry being treated give satisfactory results. Preferably, amounts from about 2 to about 4% are employed.

The quenching reaction may be conducted at temperatures in the rang from about 70 C. to about 100 C. but temperatures from about to about C. are preferred. The time of treatment can also be varied. As short a period of time as 10 minutes is beneficial and one hour is adequate in most instances although it may be practical on some occasions to allow the quenching action to continue for several hours. Preferred quenching'times are those from about 30 minutes to about 60 minutes. Air and water are preferably excluded during the quenching and recovery operations and an inert atmosphere is ordinarily maintained during these process steps.

The invention is illustrated in the following examples which, however, are not to be construed as limiting it in any manner whatsoever.

EXAMPLE 1 A series of runs was conducted in which propylene was polymerized as followsz High purity commercial n-hexane was dried and deoxygenated for use as a polymerization 4 when these additives are not used. Results of the color evaluations obtained together with the treating conditions to which the various polymers were subjected are pre sented in Table I.

Table I Color Run 011 011 811014 Temp., Tlrne No. (Wt. percent) (Wt. percent) O; (M1n.) Y Greyness Stab. Unstab Stab. Unstab.

5 None 80 5 None 90 5 1. 80 Y 5 2 95 None 90 10 None 90 10 None 95 10 l 70 10 1 90 10 2 90 10 4 95 medium. High purity propylene was dried over molecular sieves to obtain monomer with less than 4 ppm. H O. The polymerization catalyst comprising titanium trichloride and triethyl-aluminum was prepared by mixing or complexing the catalyst ingredients in hexane within a controlled atmosphere dry box. The concentration of titanium trichloride in the complex varied from about 2.4 to about 3.0 millimoles per liter while the aluminum-titanium=molar ratio was approximately 3.6. Hexane and complexed catalyst were charged to a polymerization vesselequipped with an agitator and heated ,to a temperature of about 65 C. Propylene was admitted to the polymerization vessel and the polymerization was conducted at 70 C. and a maximum pressure of 50 p.s.i.g. 'for a period of about two hours. At the end of the polymerization reaction, the polymer was in the form of a slurry (containing about solids) in the hexane diluent.

Samples of the reaction slurries (approx. 1500 ml.) were quenched by adding either methanol alone or methanol and a small amount of stannic chloride, and heating the resultant mixture to a given temperature over a certain period of time. Thereafter, the mixture was cooled to room temperature and filtered. The polypropylene precipitate was washed three times with dry methanol and dried under vacuum at a temperature of about 6570 C.

The polypropylene samples thus obtained in the form of finely divided white powders were densified and com pression-molded test pieces for color evaluation were prepared from them at 250 C. using a maximum pressure of 20 tons per square inch. The color of the pieces (0.125 in. thickness) was determined by standard spectrophotometric means which measures degree of yellowness. Reflectance (R) of the test pieces at wavelengths of 700 millimicrons, 500 millimicrons and 433 millimicrons was measured and a numerical value, Y calculated according to the formula was obtained to provide for comparative evaluation. The lower the value obtained the whiter or less colored is the polymer and the less catalyst residues does it contain. Greyness values, i.e., the reflectance measured at a wavelength of 560 millimicrons, were also obtained for each test piece. In this case,the higher values signify whiter or less colored polymer and lower catalyst residues. In some instances, the polymer was stabilized prior to color evaluation by the addition of 0.1% by weight of an antioxidant known to the trade as Ionol or 0.1% by Weight Ionol plus 0.1% by weight of stannous stearate. Values obtained in these cases are described as stabilized. Experience has shown that when such stabilizers are present, color values obtained tend to be higher in general than EXAMPLE 2 Following essentially the same procedure outlined in Example 1, ethylene was polymerized in'the presence of a catalyst comprising diisobutylaluminum hydride and titanium tetrachloride. The concentration of titanium tetrachloride in the catalyst complex was about 2-3 millimoles per liter while the aluminum-titanium molar ratio was approximately 1:2. The polymerization run was conducted at a temperature of 85-90 C. and a maximum pressure of about 100 p.s.i.g. .to produce a polyethylene reaction slurry containing from 17-18% solids.

Samples of the reaction slurry (approx. 1500 ml.) were quenched by adding either methanol alone or methanol and a small amount of stannic chloride and heating the resultant mixture at a temperature of about 90 C. for from 30 min. to one hour. Thereafter the mixture was cooled to room temperature and filtered. The polyethylene precipitate was washed three times with dry methanol and dried under vacuum at a temperature of about -70 C.

Test pieces of the polyethylene were prepared as described in Example 1 and evaluated as to color. Results of these evaluations together with the particular treating conditions employed are presented in Table II.

Table II Color Run CHaOH SnCh Time No. (Wt. percent) (Wt.percent) (Min) Y Greyness 15 None 30 31. 6 75.9 15 2.5 30 21.9 75.9 25 None 60 28.4 74.8 25 2.5 60' 10.0 78 6 The data in Tables I and II demonstrate that quenching of the catalyst in a Ziegler polymerization with an alcohol and stannic chloride more eifectively reduces the metallic. catalyst residues in the polymer than does the conventional method of quenching with alcohol alone. This is evidenced in the much better color values obtained for the molded test pieces made from the polymer recovered after. quenching with stannic chloride in the alcohol quenching agent.

The process of the, present invention is applicable in the polymerization of any. olefin. It can be employed With any ethylenically unsaturated hydrocarbon or mixtures thereof which can bepolymerized with Ziegler catalysts to produce homopolymers or interpolymers, respectively. Exemplary of such materials in addition to propylene and ethylene are'butene-l, heptene-l, octadecene-l, dodecene-l, 3-methylbutene, 4-methylbutene, styrene, vinyl cyclohexene and the like. Diolefins par ticularly suitable for interpolymerization include butadiene, isoprene, piperylene, cyclopentadiene, 1,4-pentadiene, and the like.

The process of the invention is not limited to any specific Ziegler catalyst but is applicable in the production of polymers made with any Ziegler catalyst. As mentioned previously, the Ziegler catalysts used for polymerization of olefins are those comprising the product formed from the reaction of a compound of a transition metal selected from Group IV-B, V-B, or VI-B of the periodic table of the elements with a metallic reducing agent. Preferably, the transition metal compounds employed are the compounds of titanium and zirconium with the halides being especially preferred although oxyhalides, organic salts or complexes of these elements can be used. The titanium or zirconium in the compounds employed should be in a valence form higher than the lowest possible valence. The tetrahalides are especially preferred although the dihalides, trihalides, mixtures of di-, tri-, and tetrahalides, etc., can be used. Titanium or zirconium compounds other than the halides which can be employed include alcoholates, alkoxides or esters such as titanium tetramethoxide (also called tetramethyltitanate), titanium triethoxide, tripropoxytitanium chloride, zirconium tetra-n-butoxide, or complexes such as zirconium acetylacetonate, K TiF or salts of organic acids such as the acetates, benzoates, etc., of titanium and zirconium.

Preferred as metallic reducing agents are organoaluminum compounds such as triethylaluminum, tributylaluminum, triisobutylaluminum, tripropylaluminum, triphenylaluminum, trioctylaluminum, tridodecylaluminum, dimethylaluminum chloride, diethylaluminum chloride, dipropylaluminum fluoride, diisobutylaluminum chloride, diisobutylalurninum hydride, and the like. Mixtures of the foregoing types of aluminum compounds can also be employed. The total reaction mixtures obtained in the formation of such compounds, i.e., by treatment of metal- I lie aluminum with alkyl halides resulting in the formation of such mixtures as dialkylaluminum halides plus monoalkylaluminum dihalides, termed alkylaluminum sesquihalides, are also suitable. In addition to the organoaluminum compounds, organometallic compounds of magnesium or zinc can be used. Also suitable are other reducing agents such as alkali metals, e.g., lithium, sodium, potassium; alkali hydrides, e.g., lithium hydride, sodium hydride; complex alkali aluminum and alkali boron hydrides, e.g., lithium aluminum hydride; complexes of alkali metal hydrides with boron triaryls or boric acid esters or boronic acid esters and the like.

The quantities of catalytic components can be varied considerably. The mole ratio of reducing agent to transition metal compound can be in the range from 0.3:1 to :1, on up to :1 or even higher. With an organoaluminum compound and a titanium halide the preferred AlzTi mole ratios are those between 1:3 and 5:1.

The amount of catalyst required is comparatively small. Generally, amounts from 0.1 to 5.0% by weight based on the total weight of monomer charged are satisfactory although amounts as small as 0.01% are sometimes permissible and larger amounts up to, say, 20% can be employed.

What is claimed is:

1. In a process for he producion of polyolefins by polymerization of an olefin in the presence of a catalytic amount of a catalyst comprising a titanium halide and an organoaluminum compound, said catalyst being suspended in an inert hydrocarbon diluent, the improvement providing for recovery of a polyolefin having reduced catalyst residues which comprises quenching the catalyst after polymerization has taken place and before any subsequent removal of the polyolefin from the reaction mixture by adding to said reaction mixture an alkyl alcohol containing from 1 to 8 carbon atoms and a minor amount of stannic chloride and recovering the polyolefin from the resulting mixture.

2. The process of claim 1 wherein said quenching is effected at a temperature from about to about 100 C.

3. The process of claim 2 wherein said alkyl alcohol is methanol.

4. The process of claim 3 wherein the amount of stannic chloride is in the range from about 0.5 to about 5% by weight.

5. The process of claim 4 wherein said quenching is effected over a period from about 30 to about 60 minutes.

6. In a process for the production of polypropylene by polymerization of propylene in the presence of a catalytic amount of a catalyst comprising titanium trichloride and triethylaluminum, said catalyst being suspended in n-hexane, the improvement providing for the recovery of polypropylene having reduced catalyst resides which comprises quenching the catalyst after the polymerization has taken place and before any subsequent removal of the polymer from the reaction mixture by heating said reaction mixture with methanol and from about 2% to about 4% by weight of stannic chloride at a temperature in the range from about to about C. for a period of time from about 30 to about 60 minutes and recovering polypropylene from the resulting mixture.

7. In a process for the production of polyethylene by polymerization of ethylene in the presence of a catalytic amount of catalyst comprising titanium tetrachloride and diisobutylaluminum hydride, said catalyst being suspended in n-hexane, the improvement providing for recovery of polyethylene having reduced catalyst residues which comprises quenching the catalyst after polymerization has taken place and before any subsequent removal of the polymer from the reaction mixture by heating said reaction mixture with methanol and from about 2% to about 4% by weight of stannic chloride at a temperature in the range from about 90% C. to about 95 C. for a period of time from about'30 to about 60 minutes and recovering polyethylene from the resulting mixture.

References Cited by the Examiner FOREIGN PATENTS 546,133 9/57 Canada.

JOSEPH L. SCHOFER, Primary Examiner. 

1. IN A PROCESS FOR HE PRODUCION OF POLYOLEFINS BY POLYMERIZATION OF AN OLEFIN IN THE PRESENCE OF A CATALYTIC AMOUNT OF A CATALYST COMPRISING A TITANIUM HALIDE AND AN ORGANOALUMINUM COMPOUND, SAID CATALYST BEING SUSPENDED IN AN INERT HYDROCARBON DILUENT, THE IMPROVEMENT PROVIDING FOR RECOVERY OF A POLYOLEFIN HAVING REDUCED CATALYST RESIDUES WHICH COMPRISES QUENCHING THE CATALYST AFTER POLYMERIZATION HAS TAKEN PLACE AND BEFORE ANY SUBSEQUENT REMOVAL OF THE POLYOLEFIN FROM THE REACTION MIXTURE BY ADDING TO SAID REACTION MIXTURE AN ALKYL ALCOHOL CONTAINING FROM 1 TO 8 CARBON ATOMS AND A MINOR AOUNT OF STANNIC CHLORIDE AND RECOVERING THE POLYOLEFIN FROM THE RESULTING MIXTURE. 